1. Field of the Invention
This invention relates to a method for producing a dispersion of borates of alkaline earth metals, and more particularly to a method for producing an alkaline earth metal borate dispersion in which extremely fine particles of the borate are uniformly dispersed.
2. Prior Art
Borates of alkaline earth metals have cleaning and dispersing performances, acid neutralizing performance, extreme pressure performance, anti-friction and anti-wear performances, corrosion preventing performance, and rust preventing performance, and are expected as a multi-functional additive for fuel oils and lubricating oils. Further, in the field of rust preventing paints, borates of alkaline earth metals are considered to be very promising as a rust preventing agent for non-pollutional rust preventing pigments, which agent replaces lead or chromium type rust preventing pigments.
However, when borates of alkaline earth metals are used as additives for fuel oils, lubricating oils or rust preventives, if the borates of alkaline earth metals are used in a powdered form, they are not well be dispersed but precipitate in the fuel oils, lubricating oils or solvents thereby failing to exhibit their properties because of their unduly large particle size and, furthermore, they are likely to cause friction and the like within the system. Therefore, when borates of alkaline earth metals are used as additives, they are required to be used in such a dispersion form that they are uniformly dispersed in an extremely fine particulate form without the precipitation in a dilution solvent.
As a method for producing such a dispersion of borates of alkaline earth metals, U.S. Pat. No. 3,679,584 discloses a method which comprises heating and reacting an alkaline earth metal carbonate overbased alkaline earth metal sulfonate (this term being defined in column 2, lines 9-14 of the above Patent), boric acid and an alkaline earth metal hydroxide in a mineral oil or a dilution solvent while blowing carbon dioxide gas thereinto, and U.S. Pat. No. 3,829,381 discloses a method which comprises reacting calcium carbonate overbased petroleum calcium sulfonate with boric acid in a mineral oil.
However, the dispersions of alkaline earth metal borate dispersions obtained by these methods have a low molar ratio of boron to alkaline earth metal in the entire dispersions, and also have insufficient extreme pressure performance and corrosion preventing performance. Also, since the compositions, that is, the dispersions contain the alkaline earth metal carbonate, e.g. calcium carbonate, the particles of the carbonate of alkaline earth metal increase in size as the dispersion takes a longer time to be used, and the total base number measured by the hydrochloric acid method is lower than that measured by the perchlorate method.
As a similar alkaline earth metal borate dispersion, Japanese Patent Laid-Open Gazette No. 39702/76 discloses a method for producing a mixture of an alkali metal borate dispersion and an alkaline earth metal borate dispersion. In this method, neutral sulfonate of an alkali or alkaline earth metal is allowed to react with an alkaline earth metal base and carbon dioxide gas in an inactive hydrocarbon solvent to form an overbased sulfonate, which is then contacted with 2-6 parts in mol of boric acid per 1 part in mol of an alkaline earth metal carbonate which is present as the overbased alkali or alkaline earth metal sulfonate, in an inactive oleophilic reaction medium to form an alkaline earth metal borate dispersion which is then contacted with an alkali metal base.
However, the above-mentioned method of producing a dispersion of an alkaline earth metal borate requires two reaction steps, in which a neutral sulfonate is first overbased and then allowed to react with boric acid. In addition, this method has a problem that it is impossible in the second step to freely control the calcium borate content in the thus formed dispersion of alkaline earth metal borate and that said content is limited by the content of alkaline earth metal carbonate in the overbased sulfonate produced in the first step of the reaction process.
As described above, prior art methods for producing a dispersion of alkaline earth metal borates are disadvantageous in the performance of alkaline earth metal borate dispersion obtained or in their reaction steps. The inventors of this invention made various studies in attempt to find an excellent method for producing a dispersion of alkaline earth metal borates and, as a result of their studies, found that a dispersion of alkaline earth metal borates having excellent performance can be obtained by a specific producing method. This invention is based on this finding.